CI: added missing apt update

.github/workflows/build.yml

@@ -215,6 +215,11 @@ jobs:
     - name: Checkout LFS objects
       run: git lfs checkout
 
+    # Ubuntu only:
+    - name: apt update
+      if: startswith(matrix.config.os, 'ubuntu')
+      run: sudo apt update
+
     # Ubuntu only:
     - name: Install packages
       if: startswith(matrix.config.os, 'ubuntu')